As is now well established, carbon fibers can be effectively derived from petroleum pitch as well as from other carbonaceous materials such as coal tar oils. In general, the overall process involves first treating the feed material to convert at least a portion thereof to a mesophase fraction containing from 40% to 100% mesophase. These initial procedures include solvent extraction to separate neo-mesophase or mesophase fractions. Heat treatment by itself or in combination with solvent extraction has also been utilized to obtain or to increase the mesophase portion of the feed material. The goal of these initial treatments is to obtain from the feed material a maximum amount of spinable mesophase material and also material which will give spun carbon fibers having the desirable tensile strength and Young's modulus characteristics.
Conventional spinning apparatus is employed to produce from about 500 to 3000 fibers having diameters ranging from about 8 to 15 microns. The "green" spun carbon fibers are collected in the usual manner on a spinning spool or bobbin. Since the as-spun fibers are weak and easily damaged, it has been customary to render them infusibly by a separate oxidation or thermosetting treatment step. After such a treatment the fibers are subjected to a carbonization step to convert the spun carbon fibers to usable product fibers having fixed tensile strengths and Young's modulus.
Oxidized pitch fibers are known to be easier to handle than unoxidized carbon fibers because of an increase in tensile strength. However, the present method of unwinding the "green" carbon fibers from the spinning spools and oxidizing the fibers as yarns or strands is both time-consuming and expensive in terms of the equipment needed. Thus, for example, a one pound spool of 1000 filaments contains approximately 8635 of carbon fiber. A typical commercial oxidation oven for unwinding the green fiber and for oxidizing them would be at least 50 feet in length and retention time would be one hour. Consequently, such an oxidation procedure would require at least 172 hours to process this one-pound spool of fibers. It follows therefore that there is need for other procedures whereby the oxidation or thermosetting of the fibers can be achieved in much less time and without the need to utilize elaborate and expensive equipment.
As will be understood by those skilled in this field, high strength graphite fibers produced from rayon and polyacrylonitrile (PAN) require controlled stretching during oxidation in order to obtain the orientation necessary to produce high tensil strength, carbonized fibers. Oxidation of these fibers is therefore done by unwinding of the fibers and tensioning them over rolls or godets during oxidation. In contrast, pitch fibers do not require stretching during oxidation because the orientation necessary for high tensile strength occurs during the spinning step. Nevertheless, present practice for the oxidation of pitch fibers is to unwind these fibers and pass them through a heated zone using low tension or on a conveyor belt. For 10 to 15 micron fibers an oxidation retention time of at least one hour, as discussed above, is required due to the diffusion time of oxygen into the fiber.
The need to increase the production speed of carbon fibers is recognized in a recent U.S. Pat. No. 4,351,816, to Schulz. It is interesting to note that in this patent conventional oxidizing or thermosetting procedures are followed. The delicate nature of the spun fibers is recognized, even after infusibilization, and the invention disclosed and claimed therein is directed to an improvement in the carbonization and pyrolysis treatment where breakage increases due to a loss of load-bearing capacity of the thermoset carbon fiber as it is raised from room temperature to about 700.degree. to 800.degree. C. This places a limitation on production rate.
U.S. Pat. No. 4,351,816 further reveals by implication that production rate could also be achieved by providing new procedures for oxidation or thermosetting. However, improvement in this area is more difficult than even the Schulz development for the carbonization step, since the as-spun fibers (i.e. the green fibers) are more fragile at this stage than ater thermosetting, which is what Schulz was dealing with in his procedure.
There have also been a number of prior art proposals which address the problems caused by the exothermic nature of the oxidation treatment of carbon fibers. In thes proposals a substance or mixture of substances is applied to the surfaces of the as-spun fibers prior to the oxidation or thermosetting treatment. U.S. Pat. No. 4,275,051 to Barr utilizes an aqueous finishing composition comprising a dispersion of graphite or carbon black in water. The aqueous solution also contains water-soluble oxidizing agents and surfactants. According to Barr, penetration of the graphite or carbon black particles between the filaments results in greater lubricity between filaments thereby preventing physical damage to the fiber surfaces during subsequent processing. Improved penetration of the oxidizing gas is also said to occur, which helps reduce oxidation time, exothermic excursion and filament fusions. Such fusions are highly undesirable, since they reduce the flexibility and tensile strength of the fiber products.
Aside from the need to formulate a special finishing composition and the added step of applying the finishing soulution to the as-spun fibers, the Barr procedure has the further disadvantages of adding potential impurities into the system.